Solubility refers to the ability of a substance, known as the solute, to dissociate in another substance, known as the solvent, to form a homogeneous mixture called a solution. 

The solubility of a substance is determined by the interactions between the solute particles and the solvent particles. These interactions can be influenced by factors such as the nature of the solute and solvent, temperature, pressure, and the presence of other solutes or additives.

Solubility can be reported in various units, such as grams per liter (g/L), moles per liter (mol/L or M), or as a percentage.

Substances can be classified into three categories based on their solubility:

• Soluble: A substance is considered soluble if it readily dissolves in a given solvent to form a solution. Examples include table salt (sodium chloride) dissolving in water.

• Insoluble: A substance is considered insoluble if it does not dissolve appreciably in a given solvent. Examples include sand in water or oil.

• Partially soluble: Some substances exhibit partial solubility, where they dissolve to a limited extent in a solvent. This can vary depending on the conditions and the amount of solute and solvent present.

The solubility can vary with temperature and pressure. In general, as temperature increases, the solubility of solid solutes in liquids tends to increase. However, for gases dissolved in liquids, the solubility typically decreases with increasing temperature.

SOLUBILITY RULES

There are three major rules that are the most popular in terms of solubility:

1. All alkali metals are soluble. 

Examples:

2. All nitrates are soluble. 

Examples:

3. All ammonium salts are soluble. 

Examples: 

Oftentimes, to consider if a compound is soluble or insoluble, a list of solubility rules must be consulted to make the determination.  Here is an example of a list of solubility rules. 

Solubility affects the formulation of pharmaceuticals, the design of chemical reactions, the extraction of desired compounds from natural sources, and the development of materials with specific properties.